1. Field of the Invention
The present invention relates to a process for fabricating a thin battery and a thin battery fabricated thereby.
2. Description of the Prior Art
In recent years, as portable equipments such as a notebook personal computer and a portable telephone have spread, applications of batteries with high-energy density which is represented by an Ni metal hydride secondary battery and lithium-ion secondary battery have been expanded rapidly. Conventionally, as for a lithium-ion battery, its winding body, which was obtained by winding a cathode and an anode in a laminated structure via a separator, was contained in a metallic can, and the cathode, the separator and the anode came closely into contact by means of a pressure of the can so that electric connection was obtained. However, since the metallic can was used, there arose a problem that the battery is difficulty lightened and thinned.
In order to solve this problem, Japanese Patent Laid-Open Publication No. 10-177865 (1998) or the like suggests a structure of a thin battery and process for fabricating the same which do not require a metallic can because a cathode and an anode are bonded to a separator before they are contained in a battery package. FIG. 3 are step diagrams showing one example of a conventional process for fabricating a thin lithium-ion secondary battery. A battery body 21 is composed of a wound body 22 which is obtained by winding a cathode and an anode via a separator in a laminated structure and leads 23 which are connected to the cathode and the anode, respectively. The battery body 21 as shown in FIG. 1(A) is contained in a flexible package 25 composed of a laminated film made of metallic foil and resin film, and a battery 20 is fabricated by a step shown in FIG. 1(B) of injecting electrolyte solution 26 into the battery body 21 and a step shown in FIG. 1(C) of sealing a opening of the package 25 in a state that the leads 23 are taken out from the package 25. These steps are carried out in an atmosphere of inert gas and under normal pressure.
However, when the battery 20 is left to stand in a high-temperature environment for use or preservation after sealing, a volume of gas remaining in the package is expanded, and gas is generated due to self-discharge of the battery and vapor pressure of the electrolyte solution rises. As a result, internal pressure of the battery rises, the hazard that the battery is exploded increases. Moreover, in the above process, since air bubbles remaining in porous battery materials causes insufficient filling of the electrolyte solution into the porous battery materials, paths of the electrolyte solution are blocked. Accordingly, internal resistance of the battery rises.
Furthermore, in order to make charge and discharge cycle to progress smoothly, a preparatory charging is carried out as a pre-treatment after the sealing. The pre-treatment comprises generating gas that is to be generated due to reaction between the anode and the electrolyte solution in the initial stage of a charge, before the charge and discharge cycles hereinafter. As a result, in the case where the battery is put in the high-temperature environment, the volume of gas generated due to the preparatory charge is expanded, thereby possibly causing the problem similar to the above one.
Therefore, it is an object of the present invention to provide a process for fabricating a thin battery and a thin battery fabricated thereby which can solve the above problems and are capable of filling a battery body with an electrolyte solution without expanding the volume of the battery even if the battery is put into a high-temperature environment and without involving a rise in the internal resistance of the battery.
In order to solve the above problems, the inventors have concentrated themselves on researches and found that a package is sealed at a pressure which is lower than atmospheric pressure and is reduced to not less than vapor pressure of the electrolyte solution so that expansion of the volume of a thin battery can be suppressed even when the battery is left in a high-temperature environment.
A process for fabricating a thin battery of the present invention, in which a battery body is contained in a flexible package, the battery body being composed of either a laminated body obtained by laminating a cathode and an anode via a separator or a wound body obtained by winding a cathode and an anode via a separator in a laminated structure and a plurality of leads, and being sealed in the package, is characterized by including:
containing the battery body into the package and injecting an electrolyte solution into the package; and sealing an opening of the package at a reduced pressure which is lower than atmospheric pressure and is not less than vapor pressure of the electrolyte solution. According to the present invention, since the pressure in the battery is kept at the reduced pressure which is lower than atmospheric pressure, rise in pressure in the battery due to the volume expansion of the gas in the battery, and expansion of the volume of the battery can be restrained, even if the battery is left in a high-temperature environment. The pressure at the time of sealing is preferably a pressure which is lower than the atmospheric pressure and is not less than vapor pressure of the electrolyte solution at the temperature of the sealing. In order to restrain evaporation of the electrolyte solution, the pressure at the time of sealing is preferably 1.5 times as high as the vapor pressure of the electrolyte solution, and more preferably three times as high as the vapor pressure of the same.
In addition, after injecting the electrolyte solution into the package at the reduced pressure, that is, the gas in pores of the battery body is evacuated and the electrolyte solution is injected, the package can be sealed at a pressure which is lower than the atmospheric pressure and is not less than the vapor pressure of the electrolyte solution. As a result, in addition to the above effect, since the battery body can be filled with a sufficient amount of the electrolyte solution while the evaporation of the electrolyte solution being restrained, rise in internal resistance of the battery can be prevented.
Further, it is preferable that a preparatory charging step can be included between the step of injecting the electrolyte solution and the step of sealing at the reduced pressure. Since the gas which is generated at the time of the preparatory charging can be evacuated at the time of the sealing at the reduced pressure, the volume expansion of the battery due to the rise in the pressure in the battery can be restrained even under a high-temperature environment
In addition, the package can be sealed at a reduced pressure by thermally fusing the opening.
In addition, it is desirable that the package is sealed at a reduced pressure at a temperature which is not more than room temperature, preferably not more than room temperature to not less than 5xc2x0 C., and more preferably not more than 30xc2x0 C. to not less than 5xc2x0 C.
In addition, it is preferable that a battery body, which is obtained by bonding the separator to at least one surface of the cathode and the anode, is used. Since strong metal can is not required in order to keep electric connection between the cathode, the separator and the anode, a thin and light battery without volume expansion can be provided.
In addition, a thin battery which comprises a battery body, which is composed of either a laminated body obtained by laminating a cathode and an anode via a separator or a wound body obtained by winding a cathode and an anode via a separator in a laminated structure and a plurality of leads which are connected to the cathode and the anode, respectively, and an electrolyte solution, and a flexible package for containing the battery body therein, is characterized in that the package is sealed at a reduced pressure, thereby maintaining a pressure in the package to be lower than atmospheric pressure.